Validity of a Delphi consensus definition of growth restriction in the newborn for identifying neonatal morbidity

Background Small-for-gestational age (SGA) is defined as a birthweight below a birthweight-percentile threshold, usually the 10th percentile, with the 3rd or 5th percentile used to identify severe SGA. SGA is used as a proxy for growth restriction in the newborn, but SGA newborns can be physiologically small and healthy. This definition also excludes growth restricted newborns who have weights above the 10th percentile. To address these limits, a Delphi study developed a new consensus definition of growth restriction in the newborn based on neonatal anthropometric and clinical parameters, but it has not been evaluated. Objectives To assess the prevalence of growth restriction in the newborn according to the Delphi consensus definition and to investigate associated morbidity risks compared to definitions of SGA using birthweight-percentile thresholds. Study design Data come from the 2016 and 2021 French National Perinatal Surveys which include all births ≥22 weeks and/or with birthweights ≥500 grams in all maternity units in France over a one-week period. Data are collected from medical records and interviews with mothers after the delivery. The study population included 23,897 liveborn singleton births. The Delphi consensus definition of growth restriction was birthweight <3rd percentile or at least 3 of the following criteria: birthweight, head circumference or length <10th percentile, antenatal diagnosis of growth restriction or maternal hypertension. A composite of neonatal morbidity at birth, defined as five-minute Apgar score <7, cord arterial pH <7.10, resuscitation and/or neonatal admission, was compared using the Delphi definition and usual birthweight-percentile thresholds for defining SGA using the following birthweight percentile groups: <3rd, 3rd-4th and 5th-9th percentiles. Relative risks were adjusted (aRR) for maternal characteristics (age, parity, body mass index, smoking, educational level, pre-existing hypertension and diabetes, and study year) and then for the consensus definition and birthweight percentile groups. Multiple imputation by chained equations was used to impute missing data. Analyses were carried out in the overall sample and among term and preterm newborns separately. Results 4.9% (95% confidence intervals (CI): 4.6-5.2) of newborns were identified with growth restriction, of whom 29.7% experienced morbidity, yielding a aRR of 2.5 (95% CI: 2.2-2.7) compared to newborns without growth restriction. Compared to birthweight ≥10th percentile, morbidity risks were higher for low birthweight percentiles (<3rd aRR=3.3 (95%CI: 3.0-3.7), 3rd-4th RR=1.4 (95%CI:1.1-1.7), 5th-9th RR=1.4, (95%CI:1.2-1.6)). In adjusted models including the definition of growth restriction and birthweight percentile groups and excluding birthweights <3rd percentile, which are included in both definitions, morbidity risks remained higher for birthweights at the 3rd-4th percentile (aRR=1.4, 95% CI: 1.1-1.7) and 5th-9th percentile (aRR= 1.4, 95%CI: 1.2-1.6), but not for the Delphi definition of growth restriction (aRR= 0.9, 95%CI: 0.7-1.2). Similar patterns were found for term and preterm newborns. Conclusion The Delphi consensus definition of growth restriction did not identify more newborns with morbidity than definitions of SGA based on birthweight percentiles. These findings illustrate the importance of evaluating the results of Delphi consensus studies before their adoption in clinical practice.